鄂尔多斯盆地临兴地区上古生界泥岩地球化学特征及地质意义

利用对沉积环境敏感的元素或者元素组合可以定性或半定量地恢复古沉积环境。通过鄂尔多斯盆地临兴地区上古生界31件泥岩样品的主量元素、微量元素以及稀土元素的分析测试,对沉积水体的古盐度、古气候、古水体氧化还原环境以及古物源特征进行了探讨。结果显示：①Ba元素含量以及Sr/Ba比值的变化特征反映古盐度从上石炭统本溪组和下二叠统太原组到下二叠统山西组、中二叠统石盒子组和上二叠统石千峰组明显降低,沉积环境从海相到陆相变化;②Sr含量、Mn含量、Sr/Cu和FeO/MnO指示上古生界从本溪组到石盒子组沉积期的古气候温暖潮湿,但太原组沉积期存在一段干旱期,石千峰组沉积期为干旱气候;③V/Cr、Ni/Co以及REE等对沉积水体氧化还原环境的指示性较差,通过泥岩颜色和有机质含量可推断上古生界水体由本溪组、太原组以及山西组的还原环境转化为石盒子组和石千峰组的氧化环境;④临兴地区具有明显的轻稀土富集特征,但从本溪组到石千峰组,轻稀土的富集性变差,Ce含量表现为基本正常或轻微负异常,Eu表现为"V"型逐渐变弱的负异常;⑤上古生界的CIA指数显示物源区受到中等-强烈风化。综合稀土元素分析表明,临兴地区的物源来自鄂尔多斯盆地北部的阿拉善-阴山古陆。

Geochemical characteristics and geological significance of the Upper Paleozoic mudstones from Linxing area in Ordos Basin

Paleo-sedimentary environment could be recovered qualitatively or semi-quantitatively by using elements or element associations sensitive to sedimentary environment. Major, trace and rare earth element compositions of 31 mudstone samples were analyzed to investigate the paleo-salinity, paleoclimate, paleo-redox and paleo-provenance characteristics of the Upper Paleozoic sedimentary water mass in Linxing area, Ordos Basin. The change characteristics of Ba content and Sr/Ba ratio reflected the paleo-salinity declined from Upper Carboniferous Benxi Formation and Lower Permian Taiyuan Formation to Lower Permian Shanxi Formation, Middle Permian Shihezi Formation and Upper Permian Shiqianfeng Formation, as well as the sedimentary environment varying from marine to terrestrial facies. Sr content, Mn content, Sr/Cu and FeO/MnO indexes indicated the Upper Paleozoic paleoclimates were humid from Benxi Formation to Shihezi Formation but arid in Taiyuan and Shiqianfeng Formations; V/Cr, Ni/Co, U/Th and REE were poor paleo-redox proxies of sedimentary water environments for Linxing area. Based on colors and organic contents of mudstones, it was deduced that the reducing environment in Benxi, Taiyuan and Shanxi Formations changed to the oxidizing environment in Shihezi and Shiqianfeng Formations. Light rare earth elements were obviously rich in Linxing area, but they became poorer from Benxi Formation to Shiqianfeng Formation with normal or slightly negative Ce anomalies and gradually weakening V-shaped negative Eu anomalies. The CIA values of Upper Paleozoic strata indicated that the provenance area had suffered medium-strong weathering. Furthermore, based on the analyses of REE patterns, Hf-La/Th and ∑REE-La/Yb diagrams, La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10 discrimination diagrams, it suggested that the provenances of Linxing area were mainly from Alashan-Yinshan archicontinent in the northern Ordos Basin.